If not, any suggestions for a free FPS display program (besides NVIDIA's GF Experience)?

MSI Afterburner is a great program to show FPS and temps and so on. Use FRAPS if you want no effort for setting up a counter.

To be honest stick with MSI and install the rivia tuner when it asks during the install process. it gives you so many options and you get a tool to overclock your graphics card and tune fan curves to increase cooling for your card.

Scroll to bottom for download
https://www.msi.com/page/afterburner

Fraps here for simple FPS counter with no effort to set up.
http://www.fraps.com/

Ctrl+Shift+Tab enables it.

Galaxy has a FPS counter? never knew this was a thing. well OP ignore all my shit then unless you don't use Galaxy

You learn something new every day. I'll have to give it a try once I get home.

Thanks May not be a waste since it appear that some games just won't display FPS, mainly the games where the Galaxy in-game hub doesn't work (Shift+Tab).

Will Afterburner work with non-MSI cards?

Works fine on my NVIDIA 940MX.

I'd definitely recommend MSI Afterburner (also works with non MSI cards). Aside from an Min / Avg / Max FPS counter, the On Screen Display can be customised and includes things like 1% and 0.1% lows and CPU usage by core which is a lot more helpful when diagnosing micro-stutter in games with low fps spikes but otherwise high avg fps.

You can install only RTSS Rivatuner Statistics Server if you don't want/need MSI Afterburner. It also has a very useful FPS limiter.

You might want to install FRAPS too, as a backup FPS monitor. In rare occasions some games won't show the FPS with Rivatuner and other may display graphical glitches. It was the case with Brigador - the pilots portraits were glitched while Rivatuner was running. But it might be fixed in newer versions, as I played the game like a year ago.

+1 for MSI afterburner / Riva tuner SS. Awsome free program that helps tune the game for best smooth experience (very difficult to get right on low end devices).
I had games crashing on launch with MSI running as well, But overall, is by far the program I use the most for games (it does GPU overclock too, use it on desktop to under-clock and under-voltage)

Edit: just wish something similar works on linux distros.

I´ve tried with old games and don´t seem to work (populous TB and Dungeon Keeper). Does it only work with "modern-ish" titles or I am doing something wrong here?

I didn't know this either, GOG should really put it in Settings or something so people know it actually exists.

Try shift+tab to open the Galaxy overlay. If that doesn't work the fps counter won't either.
I think it shows somewhere on the overlay once that's open?